1. Field of the Invention
The invention resides in the field comprising the electrical transfer of electrolytes (including weakly dissociated electrolytes) from a first fluid to a second fluid and more particularly relates to apparatus and processes utilizing the principle of electrodeionization(EDI), i.e. to electrodialysis apparatus and processes in which at least one compartment of said apparatus is packed with particulate ion exchange material.
2. Description of the Prior Art
Electrodeionization (EDI) is a process for transferring electrolytes (including weakly dissociated electrolytes) from a first fluid to a second fluid under the influence of a substantially direct electric potential applied to an electrodialysis apparatus in which at least some of the flow compartments, generally at least some of the diluting compartments, are substantially filled or packed with particulate ion exchange material.
EDI is particularly attractive to deionize waters with low total dissolved solids content, which exhibit low electrical conductivity and which thus limit the current capacity of conventional electrodialysis. The highly conductive ion exchange resin packing in EDI provides a conductive path between ion exchange membranes and also increases the surface area available for removal of ionized dissolved solids from the water.
Although it is not intended that this invention be limited in any way by any theory of electrodeionization (i.e., electrodialysis with compartments packed with particulate ion exchange material) nevertheless the following may help to understand the technology, this invention, and the various functions of the particulate ion exchange material packing.
Referring to FIG. 1(a) it represents schematically and in a simplified way one possible arrangement of such packing. In 1(a) "C" represents a cation selective membrane, i.e. an electrolytically conductive sheet or film in which the electric current is carried substantially exclusively by cations; "A" represents an anion selective membrane, i.e. an electrolytically conductive sheet or film in which the electric current is carried substantially exclusively by anions; "CX" represents particulate cation exchange material and "AX" represents particulate anion exchange material. The mix of particulates may range from all-cation-exchange to all anion-exchange, but for electrodeionization approximately an equinormal mixture of CX and AX is generally preferred. For purposes of illustration in FIG. 1(a) the space between membranes C and A is packed with an approximately equinormal mixture of particulate CX and AX, in one possible random arrangement. There are many different such arrangements of the 8 CX particles and 12 AX particles corresponding to this approximately equinormal example based on typical commercial ion exchange resin capacities, but that shown will serve to illustrate most of the processes occurring in such packed deionization cells.
The chamber represented by the region between membrane C and membrane A is a demineralizing, depleting, or diluting compartment, i.e. if the compartment contains in addition to the ion exchange particulates CX and AX a solution of a dissociated electrolyte in the interstices among the particles CX and AX, then cations from such solution will tend to migrate through membrane C toward electrode E.sup.- and anions through membrane A toward electrode E.sup.+ thereby depleting the electrolyte in the chamber. It is well known (See Heymann and O'Donnell, J. Colloid Sci., 4, 395(1949) ) that strongly dissociated ion exchange materials (including ion selective membranes) have specific electrical resistances of order of magnitude about 100 ohm-cm, i.e. about the same as an aqueous solution containing about 0.1 gram equivalent of sodium chloride per liter. Hence if the solution in the interstices is an aqueous solution of sodium chloride containing much less than about 0.1 gram equivalent per liter (say about 0.01 gram- equivalent per liter) then the electrical resistance of the ion-exchange particulates will be much less than that of the solution. Then the low resistance path for anions will be through anion exchange particulates AX and for cations through cation exchange particulates CX. The first column from the left is such a low resistance cation path. The cation exchange particle in the second column also feeds that path. In the two dimensional pattern of FIG. 1(a) the cation exchange particle in the fourth column is a dead end (but in three dimensions, i.e. in the planes immediately above and below that shown in FIG. 1(a), there could be connections to CX particle paths leading to membrane C. There are no dead-end anion particles in the figure. Some anions pass around cation exchange particles which are in the way. At many contacts between the particles it is possible for a cation to enter a cation exchange path and simultaneously for its "companion" anion to enter an anion exchange path. One can postulate that when the electric current passing through the electrolyte solution in the interstices between the particulates and through the particulates is such that the voltage drop across the interfaces between particulates AX and membrane C and between particulates CX and membrane A approaches a certain threshold voltage value (probably about 0.3 volts in the case of most commercially available anion exchange particulates and anion selective membranes) dissociation of water into hydrogen ions and hydroxide ions will occur at such interfaces, possibly catalyzed by weakly dissociated moieties. In this case, at membrane C, hydrogen ions will pass into the membrane and hydroxide ions will tend toward anode E.sup.+ through anion exchange particulate paths. At membrane A hydroxide ions will pass through the membrane and hydrogen ions will pass into cation exchange particle paths and tend toward cathode E.sup.-. Similarly hydrogen and hydroxide ions can be formed at the junction between the CX and AX particles in the fifth column in the figure as well as between the CX particle in the second column and the AX particle immediately below such CX particle.
Such packed electrodialysis apparatus, operating at current densities which result in generation of hydroxide and hydrogen ions, may be regarded as continuously, electrically regenerated mixed bed ion-exchange deionizers. Cost effective apparatus and processes may be achieved by a judicious choice of ion exchange particles with regard to resin type, particle size and shape, and anion-to-cation ratio and relative positioning in addition to selection of the optimal combination of equipment design and operating process parameters.
Although FIG. 1(a) suggests that particulates AX and CX are beads or spheres they can in fact be any structures which provide fluid interstices and permit flow of such fluid in the interstices, for example irregular granules, thin rods preferably parallel with the surfaces of the membranes, fibers including woven or knitted fibers, saddles, rings, tellerettes, etc. For purposes of this invention beads, spheres, or other granules are highly preferred.
Other possible arrangements of ion exchange particulates in the deionization cells are possible. For instance, FIG. 1(b) illustrates schematically and in a simplified way another possible arrangement of such packing. In this case the low resistance path for anions will be through the anion exchange particulates AX. Cations will be constrained to migrate through the fluid in the interstices between the particulates. The interface between the particulates AX and membrane C will have the possibility of formation of hydrogen and hydroxide ions at that interface, when the applied current is such that the voltage drop between the particulates AX and the membrane C approaches a certain threshold value, as already discussed in connection with FIG. 1(a). Because of the superior conductivity of hydrogen ions relative to other ions, this type of configuration is more suited to acidic fluids, including weakly dissociated acids. The faster hydrogen ions will move through the fluid, and the anions will move through the particulates AX.
FIG. 1(c) represents another possible arrangement of the particulate packing. "S" represents a thin, highly foraminous sheet such as a plastic screen or expanded plastic sheet having openings sufficiently small to prevent contact between the CX and AX particles, but permitting the flow of fluid within and parallel to the plane of the sheet in at least one direction, e.g. from right to left in FIG. 1(c); E.sup.- represents a negatively charged electrode, i.e. a cathode in electrolytic communication with membrane C through electrolytic solution(s) and/or other membranes and/or ion exchange particulates; E.sup.+ represents a positively charged electrode, i.e. an anode similarly in electrolytic communication with membrane A through electrolyte solution(s), other membranes and/or ion exchange particulates. The compartment represented by the region between membrane C and membrane A is a demineralizing, depleting, or diluting compartment.
The system of juxtaposed particulates AX and membrane A on the one hand and particulates CX and membrane C on the other hand will each behave essentially as equipotential extended surfaces, i.e. as membranes having extended surfaces, when the solution in the interstices and in the screen openings contains much less than about 0.1 gram equivalent per liter of electrolyte, because the electrical resistance of the ion-exchange particulates will be much less than that of the solution.
The compartments adjacent to the deionization chambers in the illustrations of FIG. 1 need not be packed with ion-exchange particulates in non-reversal EDI, because the electrical conductivity of the more concentrated solution in those compartments will be much higher than in the deionization compartments. For EDI a screen support in the concentrating compartments is usually satisfactory. When using symmetrical polarity reversal, in which the compartment functions alternate between deionization and concentration, the compartments should all be packed.
The concept of electrodialysis apparatus containing mixed bed ion exchange particulates in deionization compartment was apparently first disclosed by Kunin, et.al. ("Ion Exchange Resins", Wiley, New York, 1950, p 109) but no data were given. Walters, et.al. (Ind. Eng. Chem., 47, 61-67 (1955) and "Ion Exchange Technology", eds. Nachod and Schubert, Academic Press, New York, 1956) were apparently the first to disclose operating data. Other early disclosures were made by Glueckauf, et.al. (e.g., Second United Nations Conference on Peaceful Uses of Atomic Energy, Paper 308 (1958) and Brit. Chem. Eng., 4, 646-651 (1959) ). Kedem, et.al., disclosed filled cell electrodialysis in which the dilute compartments were filled with various knit ion exchange fibers (Desalination, 16, 105-118 (1975) ); such cells in the form of a tank having sealed concentrate compartments, the open dilute compartments being filled with granular anion exchange resin "which can be poured in and pumped out" (Desalination 24, 313-319 (1978) ). In the latter publication the open dilute compartments may also contain knit cation exchange fibers against the cation selective membranes. The flow of fluid through the dilute compartments was by gravity which limited the flow rate and compartment size to uneconomic values. The apparatus had the advantage that it could be easily filled with particulate anion exchanger and such exchanger could be easily removed for cleaning or replacement. The concentrate compartments depended solely on electrical transfer of water through the surrounding membranes. As a result the concentrate was in fact very concentrated and subject to scaling and precipitation of poorly soluble electrolytes. The same author(s) reported on similar electrodialysis stacks in which the dilute chambers were filled solely with a net of multifilament anion exchange material (Desalination 46, 291-299 (1983) ).
There have been many patent publications concerning packed cell electrodialysis including the following U.S. Pat. No.: 2,689,826; U.S. Pat. No. 2,815,320; U.S. Pat. No. 3,149,061; U.S. Pat. No. 3,291,713; U.S. Pat. No. 3,330,750; U.S. Pat. No. 3,515,664; U.S. Pat. No. 3,562,139; U.S. Pat. No. 3,686,089; U.S. Pat. No. 3,705,846; U.S. Pat. No. 3,993,517; U.S. Pat. No. 4,284,492; U.S. Pat. No. 4,632,745; U.S. Pat. No. 4,747,929; and U.S. Pat. No. 4,804,451. Nevertheless, although electrodialysis with packed cells (i.e. electrodeionization) has been known and studied for almost 40 years it has not yet received widespread commercial use. The reasons for this appear to be one or more of the following:
a) the need to fill individual compartments with ion exchange resin particulates, and the need to keep the resin in place while assembling the EDI stack, and the practical difficulties of doing this, especially for relatively large EDI stacks. Until now practical external filling and removal of the resin particulates has not been done because of lack of:
1) an EDI stack designed to be filled with resin after the stack has been assembled; and PA0 2) a process to fill and empty such an assembled stack, by pumping a resin slurry into or out of the stack.
b) the particulate ion-exchange packing is a very good filter medium. The resistance to flow of fluid through the packing is increased by material filtered out during operation. In the case of conventional (chemically regenerated) ion exchange deionization, the ion exchangers are periodically backwashed at flow rates which expand the volume of the particulates, i.e. separating the particulates slightly from each other allowing filtered material to escape. Until now such bed expansion capability has not been a feature of electrodeionization apparatus. Instead electrodeionization has been preceded by fine filtration. The latter is nevertheless seldom completely effective.
c) anion exchange particulates tend to sorb negatively charged colloids and medium molecular weight anions which occur naturally in water. Such sorbed materials (generally termed foulants) interfere with the satisfactory operation of the apparatus, e.g. by increasing the electrical resistance and decreasing the rate of transport of ions to the particulates. In the EDI process the electric current tends to drive such foulants into the anion exchange particulates and thereby accelerate the fouling. Until now EDI stacks have in practice been preceded by scavenging type anion exchange resin and/or activated carbon columns to attempt to remove foulants before they can enter the stacks. Such pretreatment is costly and is seldom completely effective especially in view of often unpredictable breakthroughs of foulants on column exhaustion.
d) precipitates of sparingly soluble inorganic compounds (e.g., calcium carbonate, magnesium hydroxide, calcium sulfate) tend to form within the particulate packing, in the anion exchange membranes, or in the concentrate compartments of the electrodeionization apparatus if precursors of such compounds are present in the fluid processed. Such problem does not exist in conventional ion exchange deionization in which the anion- and cation-exchange particulates are separately regenerated with alkali and acid respectively. In conventional electrodialysis such precipitates are prevented by frequent, regular reversal of the direct electric current, e.g. a few times per hour.
e) at the water dissociating junctions between commercially available anion exchange bodies (i.e. membranes and particulates) and cation exchange bodies, quaternary ammonium moieties (the usual bound positively charged group in commercially available anion exchange bodies) are rapidly converted to tertiary amines and/or non-ionized groups resulting in increased electrical resistance at such junctions. Such conversion may be due to some combination of high alkalinity, high temperature, and high electric field in the junctions. There is not an equivalent phenomenon in conventional ion exchange deionization under normal process conditions. In the case of electrodeionization until now it has been necessary after some months to a year or so to disassemble the packed electrodialysis stack and replace at least the anion selective membranes and preferably also the anion exchange particulates. Some electrodeionization stacks are sealed (i.e. the membranes and filled inter-membrane spacers are glued together) in which case it is necessary to replace the entire stack except for the screen-filled concentrate spacers;
f) the electrical resistance of the packing depends also on the area of contact of the beads, hence on the deformability of the beads, the force causing such deformation, the distribution of bead sizes and any time dependent relaxation of the force, e.g. from cracking of the beads. The overall effect is usually a time dependent increase in electrical resistance requiring eventually repair or replacement of the electrodeionization stack. A similar problem does not exist in conventional ion exchange deionization as there is no electric field.
g) owing to the short distance packed electrodialysis apparatus (e.g. about 0.3 centimeters) substantial channeling of processed fluids can occur resulting in less than expected performance.